Suspension-bridge



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JOSEPH TOMLINSON, O E PUTNAM, IOWA,

SUSPENSION-BRIDGE.

Specification of Letters Patent No. 32,440, 4dated May 28, 1861.

To all 'whom it may concern:

Be it known that I, JOSEPH TonLINsoN, of Putnam, in the county of Linn and State of Iowa, 4have invented a new and Improved Method of Constructing Suspension- Bridges; and I do hereby declare that the following is a full, clear, and exact description thereof, reference being had to the accompanying drawings and to `the letters thereon.

The nature of my invention consists in the adaptation and combination of a rigid truss to the ordinary wire cables of` suspension bridges, in such manner that the changes resulting from variations of temperature shall have no tendency to produce any undue strain between the diiferent parts of the whole structuremthe increased or reduced camber of the truss caused by variations of temperature acting upon it, always corresponding to the rise or fall in the cables from contraction or expansion.

The use `of the ordinary truss necessary to secure uniform surface to the flooring or track of a suspension brido'e designed for heavy traiiic involves a diculty of great importance. The greatest effect of change of temperature is at the center of the cable; the length of the back stays remaining constant. In bridges of from 400 to feet span the platforms att-he center' must rise from 18 to 24 inches to accommodate themselves to the shortening of the cables due to changes of temperature equal to 130 degrees. Under these conditions the more rigid the truss the less it adapts itself to the contraction or expansion of the cables, and only follows these by a partial dislocation of its parts or actual fracture. Hence the difliculty of using a truss of suiiicient strength to effectively distribute the load in suspension bridges as ordinarily constructed. For these reasons engineers of the highest reputation have condemned suspension bridges for railway traffic. In the only large structure of the kind in use the objections stated create much diificulty, preventing high rates of speed and requiring frequent adjustment. Vith the improved trusses no variation of temperature will affect the relative position or bearing of any portion of the trusses or the cables. The two systems, disconnected, will rise and fall equally with the changes or variations in the length of their respective parts; and when connected they will rise and fall `through the whole range of their deflections without any change being made in the proportion of the strain carried by each member of the structure-the two systems being' in perfect adjustment under all conditions of temperature.

The improvement consists in the employment of di'erent materials in the top and bottom chords so combined that change of temperature will-by its own action produce a curve or camber in the trusses exactly equal to the change effected in the deflection of the cable from like cause.

If the top and bottom chords of a truss be framed so that each shall be segments of a circle and parallel to each other, it is evident that the radius of the top chord will be longer than that of the bottom chord by the height of the truss. Hence it is, also, evident that if the lengthof either the top or bottom of the panels is altered the curvature of the whole structure is affected. If the bottom'chord is shortened, the difference in the lengths of the segments corresponds exact-ly with the difference of the radii of the circles that describe these segments, and as the bottom chord diminishes in length the radius of the curve becomes less and the versed sine or camber of the bridge greater.` Vith a given height of truss, therefore, the increase in t-he camber of the bridge due to the lengthening or the shortening of the chords is uniform and constant with equal increments of temperature.

lVhen the materials composing the top chord expand and contract less than the material composing the bottom chord, by varying Athe height of the truss, or the distance between the chords, any required amount of camber may be obtained from the differential contraction of the materials of which the top and bottom chords are composed; the greater the distance between the chords the less the camber given to the truss, and vice-versa. The expansion of bar-iron, for instance, under a given increase of temperature, is about three times as great as that of whitevpine. It, therefore, follows that a truss with a top chord of pine and a bottoni chord of wrought-iron will vary its curvature according to temperature, and if supported only atthe ends, its camber will be increased or diminished in ratio to the increased or diminished temperature to which it may be subjectedthe exact ratio of the rise and fall being determined by the length of the truss and the distance apart of the top and bottom'chords.

The degree of change from temperature in the elevation or depression of the cables of a suspension bridge depends upon the degree of their curvature-the greater the curvature the less the change, and vice-versa. With the same distance between the towers, therefore, by increasing or decreasing the versed sine of the catenary the absolute rise and fall in the center due to variation of temperature, may be altered to suit the requirements of the situation, or the arrangement of the truss. It is, therefore, perfectly practicable so to proportion the trusses and the cables that under vall variations of temperature they will be in perfect adjustment and work together, rising and falling uniformly with each variation of temperature, and remaining always in as exact adjustment as any other description of suspension bridge when in its normal position. To illustrate this more fully the drawings forming part of this specification show a design for a bridge of 450 feet span between the towers, constructed on the improved principle: Figure 1 being an elevation, Fig. 2 a front elevation of the towers, Fig. 3 a section of a truss, Fig. 4 a plan of the chords and braces, Fig. 5 a sect-ion of the parts enlarged, and Fig. 6 a diagram representing the changes of the cable and truss by a variation of 130 degrees of temperature. The deflection in the cable, at the highest assumed temperature, is 40 feet and its total length including both back stays 700 feet. The length of the truss is 420 feet, the height 18 feet, and the camber 1.5 feet.

The lowest line in Fig. 6, marked a represents the line of the horizon; the curved line above it, marked b shows the position of the bottom chord (iron) of the truss, the line marked c the curve of the cable; the line marked Z the top chord (pine) of the truss; all parts being in their normal position, at the temperature assumed. Under these conditions the length of the cable between the towers, will be 459.38 feet; its deflection 40 feet; the versed sine of the truss 1.5 feet, the number of panels 35; the length of each panel at the top 12.0147 feet and at the bottom'12 feet. If now we suppose a decrease of temperature equal to 130 degrees, all these dimensions are varied and the parts assume the position represented by the dottedlines above b, c, and L -'the contraction of the cable being equal to .62426 of a foot; the length between the towers 458.75574 feetthe deflection is reduced to 38.625 feet; and the platform is raised in the middle 1.37 5

feet, and in corresponding proportions along its entire length. In the meantime the truss has undergone a similar contraction, the bottom chord having contracted altogether .38 of a foot; the length of each top panel of white pine is now reduced .0035429 of a foot, while the bottom panel of wrought iron is diminished .010888 of a foot; the length of the top and bottom of each panel being now 12.011158, and 11.989 feet respectively. The effect of this is to reduce the radius of the truss from 14.700 to 8.254 feet; the versed sine being increased from 1.5 feet, its norm-al state, to 2.666 feet. Another' change besides the simple rise of the trusses and cables has taken place the effect of which has also to be considered. The suspension rods- Athat support the roadway and connect the trusses and cables have, also, contracted, and at the same time have been elevated by the movement of all parts of the cables toward the towers. By these combined movements the Suspenders are not only shortened but their direction is rendered more oblique. The long Suspenders near the towers, in which this contraction is more marked, have raised the ends of the truss .209 of a foot, making the total rise of the truss in the center 2.666, plus .209 feet, equal 2.875 feet; of this elevation 1.166 feet being due to the alteration of the truss itself-the two systems having been raised, from the nature of their materials, exactly the same distance, and remaining, consequently, in exactly the same rrelative position to each other as when placed in adjustment. Y

The improved truss is equally adapted to radiating rods or cables.

By the use of cast or wrought iron in the towers the vertical change of position at the ends of the trusses could be so reduced as to become of little or no practical importance, allowing them to rest solid upon the wall plates, allowance being made for the free movement of the bottom chords as they eX- pand o1' contract in length.

lVhat I claim as my invention and desire to secure by Letters Patent is- The within described method of constructing suspension bridges, the same consisting in combining rigid trusses having chords of material of dierent expansion and contraction under the same degrees of temperature with the wire suspension cables, substantially as herein set forth.

This specication signed this 26th day of February 1861.

JOSEPH TOMLINSON. lVtnesses H. V. Poon, JN0. H. SGHULTZ. 

